The principal effort during the next year will be to identify the amino acid residues in aminoacyl-tRNA synthetases that have reacted with specific reagents. In addition to the arginine and lysine enzymes, we plan to use the alanine and phenylalanine enzyme that can be produced in very large quantities in certain strains of E. coli. The primary sequences of these enzymes are now being determined in another laboratory by analysis of the DNA of the genes. This information should greatly facilitate our work. Reagents that we have found to react specifically to inactivate synthetases include: butanedione that reacts with arginine residues, pyridoxal phosphate that combines with specific epsilon-amino groups of lysine residues, oxidized ATP that reacts with amino groups in the binding sites for ATP and diethyl pyrocarbonate that derivatizes the imidazole ring of histidine. Conditions will be determined that give the most favorable ratio of inactivation to incorporation of reagent. The modified protein will be hydrolyzed enzymatically and with CNBr to produce large peptides. These will be separated by ion-exchange chromatography and the fragments containing the modified amino acids will be isolated and further characterized.